The present disclosure is directed to test data evaluations which are obtained in testing after an oil well has been drilled. During the drilling of a well after certain formations have been penetrated, a particular formation of interest will be tested to determine if it has sufficient porosity to produce valuable petroleum products. Present procedures involve the use of a formation tester (FT) which is typically lowered on a wireline into the open hole. This is a device which incorporates a snorkel which is extended in a known fashion into the formation to expedite formation fluid flow. The FT encloses one or more sample chambers which are filled. A valve is open in the FT to introduce measurements associated with this activity include the measurement of pressure both before and after the test, elapsed time required to obtain a specified volume of test fluid, fluid viscosity and other variables. The present procedure utilizes certain measurements obtained from the FT which yield an instantaneous indication of formation permeability.
In times past, permeability calculations ordinarily were processed only after the FT had completed the various testing and sampling procedures. Typically, the post sampling calculations of data were performed at or near the well site after removal of the FT, or were performed much later. Determinations of permeability after removal of the FT from the open hole well created a great deal of risk. For instance, data was normally obtained from the uncased well borehole before casing or other completion procedures were initiated. If too much time passed, leaving the well borehole in the open condition was viewed as risky. On the other side of the coin, if the permeability could be obtained while the FT were still in the borehole, additional tests or more accurately determined completion procedures could then be implemented. Therefore, the time lag in obtaining a final permeability calculation from data derived from a first run was so great that the delay was substantial and risky. Regrettably, the FT, once removed from the well, typically is removed from the well site. Moreover, delays in completion of such testing are expensive. Delays are expensive because the well is maintained in an open hole condition which runs some risk of sidewall collapse or directed circulation to various formations penetrated by the well, and it also ties up rig time. Rig cost is normally proportionate to time. Accordingly, if substantial time elapses between first use of an FT, subsequent determination of formation data necessitating a second use of the FT after removal, a substantial cost increment is incurred.
The present apparatus is a system, and related method, as will be described, for obtaining measurements dynamically in the downhole context so instantaneous determinations of permeability can be obtained. This involves conducting a pretest, measuring the flow rate, measuring the duration of the pretest, and also taking pressure measurements before and after the pretest whereby permeability values can be calculated by means of a computer at the surface. Other data used in this determination relate to making the Horner Plot. The present method and apparatus enable an instantaneous indication of permeability from the several variables, even while the FT is in the well and while rig time is held to a minimum.
The present disclosure is therefore directed to a method of obtaining sufficient data to mathematically compile a Horner Plot and to make other measurements so that permeability can be calculated while a formation test device is in the well borehole. This cuts down on wasted rig time. Instantaneous indications of permeability can enable decisions to be made immediately.